Method of electropolishing small metal objects



Sept. 13, 1966 J, JUMER 3,272,729

METHOD OF ELECTROPOLISHING SMALL METAL OBJECTS Filed April 10, 1963INVENTOR JOHN F. JUMER BY 8 a ATTORNEY United States Patent C) 3,272,729METHOD OF ELECTROPOLISHING SMALL METAL QBJEQTS John F. Jurner, 16 W. 131Timber Trails Drive, Elrnhurst, Ill. Filed Apr. 10, 1963, Ser. No.272,043 4 Claims. (Cl. 204140.5)

My invention has reference to a method of electropolishing quantities ofsmall metal objects.

The electropolishing of small metal parts, particularly those formed ofstainless steel, is frequently an expensive operation due to the needfor suspending each individual piece on a rack which is subsequentlyimmersed in a suitable bath. In some instances, the rack must bespecifically fabricated to receive the objects being treated and haslittle application to other work of a similar nature.

One of the principal objects of my invention is to provide a suitablemethod for electropolishing large numbers of small metal objects in bulkand thereby eliminating the need for racks to support such objects forimmersion in an electropolishing bath.

More specifically the important objects of my invention includepracticing the new method by means of an electropolishing deviceincluding a barrel adapted to contain these parts together with a drivefor this barrel adapted for periodic intermittent movement of the barrelto produce tumbling of the contained objects, separated by periods ofquiescence during which effective electropolishing occurs.

Another important object of my invention is the provision of a processwhereby large quantities of small metal objects may be given a lustrousfinish of superior quality at low cost by electropolishing withoutrequiring the racking or individual suspension of each metal object.

Additional important objects of my invention will be apparent from thefollowing detailed description and from the drawings in which:

FIGURE 1 is a partially sectioned elevation of apparaq tus suitable foruse in the practice of my invention; and

FIGURE 2 is an end view of the apparatus shown in FIGURE 1.

In brief, my method for electropolishing utilizes a barrel or drum ofgenerally polygonal cross section, preferably having not less than threenor more than eight sides, adapted for rotation about its major axis andimmersible in a liquid bath of suitable composition. The small metalparts to be electropolished are placed in the barrel in electricalcontact with the positive pole or anode of an electrical circuit. Theother pole or cathode of the electrical circuit is axially disposedwithin the barrel and spaced from the objects to be polished in suchmanner that current will flow between the parts and the cathode when thedrum is immersed in the bath.

Electropolishing metal objects is only superficially similar to plating.One of the principal distinctions between these processes is theapparent mechanism of the process. In plating, the reaction startsvirtually immediately upon completion of the circuit but inelectropolishing, an appreciable period of time elapses before thedesired reaction reaches a reasonable level of efliciency. During thisperiod the formation of a film, presumably gas, on the surface of theobjects being electropolished will be observed. Under proper conditionsthis film will initially be uniform and relatively thin and forms on thesurfaces of the objects in line of sight from the cathode. As thereaction progresses, however, bubbles of appreciable and irregular sizewill be formed in the film. These gradually enlarge and eventually riseto the surface of the bath, frequently traveling along the surface ofthe object being polished. These bubbles, if alllowed to remain in con-"Ice tact with the surface for an appreciable period of time, or to movealong the metal surface, will usually produce a streaked or mottledappearance rather than a desired uniform lustrous finish. Furthermore,they markedly increase the resistance to the flow of current.

These undesirable effects are avoided in my invention by allowing theparts in the barrel to remain motionless for a predetermined period oftime. The period of time is of importance and is selected to permit theformation of the desired film without which satisfactoryelectropolishing cannot be accomplished plus an additional period oftime in which the actual electropolishing occurs. However, before thebubbles can grow to an undesirable size or break free from the surfacebeing polished in appreciable quantities, I rotate the drum, thustumbling the parts, destroying the gas film and discharging the bubbleswhich have grown within the film without damage to the surface. This isfollowed by another motionless period and again by tumbling, the processbeing repeated as described until the desired finish is obtained.

Referring to the drawings, my apparatus includes a frame generallydesignated 10 suitably formed of a generally rectangular plate 11 havingdownwardly extending legs 12 and 13 aflixed intermediate itsextremities. The lower extremity of the leg 12 is formed to receive andsupport the upper portion of a bushing 17 which is firmly clamped to theleg 12 by a cap 18 pivotally secured to leg 12 by a pin 19. A thumbscrew 21 or other suitable device may be utilized to secure the oppositeend of cap 18 to the leg 12. The lower extremity of leg 13 is alsoformed to receive the upper half of a generally cylindrical bushing 14of suitable synthetic resin inert to the bath ingredients and of suchcharacter to provide a satisfactory bearing surface. Bushing 14 isjournaled in a metal cap 16 similar to cap 18 and attached to the leg 13in a similar manner.

A drum or barrel, generally designated 22, is formed of end members 23and 24 connected rigidly by a plurality of axially extending bars 26.The bars 26 are formed to receive and support panels 27 containing aplurality of openings somewhat smaller than the smallest metal ob jectintended to be placed in the barrel 22. In order to obtain satisfactoryresistance to chemical attack as well as relatively low cost, I preferto form the abovementioned portions of the barrel of a suitablesynthetic resin such as polypropylene. One of the panels 27 is providedwith a suitable spring clip 28 adapted to retain it in position, therebyforming a door, designated 29, through which parts may be inserted orremoved from the barrel 22 The end member 23 is shaped for engagementwith a grove 31 formed in the bushing 17 and is adapted to rotatetherein. End member 24 is somewhat larger in diameter than end member 23and is provided with a grooved periphery for engagement with a V-belt32. A speed reducer 33 driven by a motor 34 is mounted on the frame 10and drives a pulley 36 over which the belt 32 extends.

Within the barrel 22 are longitudinally extending strips 37 of asuitable metal such as titanium prefer-ably arranged on the flat innersurfaces of the panels 27 and fixed against movement by engagement withthe end member 23. These strips preferably are of such shape as toconstitute both flights to insure tumbling of the parts during rotationof the barrel and satisfactory electrical contact for the parts beingpolished.

A titanium plate 38 is secured to the strips 37 in suitable manner suchas by screws 39 and constitutes an additional contact for metal objectswithin the barrel. The central portion of the plate 38 is bored toreceive and support a bearing 41 of suitable synthetic resin such \aspolypropylene. I

A shaft 42 having a flange 43 supports the drum for rotation in thebushing 14 which is suitably bored to receive the shaft 42. The bolts 44extend through the end member 24, the flange 43, the plate 38 and thebearing 41, anchoring the component parts. A copper cathode 46 extendsaxially through the bushing 17 along the axis of the drum 22 and issupported at its free end by the bearing 41.

Insulators 47 secured to the frame by bolts 48 support grooved metalbars 49 adapted to rest on bus bars (not shown) arranged above asuitable bath in which my device may be immersed. A conductor 51connects the bar 49, intended to contact the negatively charged bus,with the cathode 46. Conductor 52 connects the other bar 49 with a brush53 mounted in a holder 54 and urged by a spring 56 into rubbing contactwith the shaft 42.

The motor 34 is connected to a suitable timing device, generallydesignated 57, of conventional design adapted to regulate the durationof the period in which the motor is actuated as well as the intervalbetween actuations of the motor. The timer 57 may for example include atiming device 58 for adjustably regulating the period of time the motor34 will be actuated. A dwell timing device 59 may also be provided toadjustably regulate the interval between motor operating periods. Sincemy device is intended to be transported by an overhead hoist, I providelifting means such as an eyebolt 61 engaging the frame 10 above thecenter of gravity and a conventional flexible cord and plug 62 forconnection with a suitable A.C. circuit after the device has been placedin position in the polishing tank.

In practice the requisite quantity of small parts to be polished isinserted in the barrel 22 through the door 29, the quantity selectedbeing such that contact with the cathode 46 will not occur duringrotation of the drum. When fully charged, the small parts form agenerally rectangular zone within the drum 22 below the cathode 46, thiszone being generally maintained throughout operation of my device. Theentire assembly is then transferred to a tank containing a bath ofsuitable composition and temperature and is supported therein by busbars which engage bars 49. The plug 62 is connected to a suitablecircuit, the timers 58 and 59 set to the desired intervals and thestarting button is pressed. The parts within the barrel are in contactwith the metal end plate 38 and with the strips 37 and thus constitutethe anode of an electrical circuit extending through the bath to thecathode 46. As previously noted, it is necessary that the drum remainmotionless for a period of time sufficient to insure the formation of asatisfactory film primarily along the surfaces of the mass of partsexposed to the cathode. This will require a minimum of approximately twoseconds and a maximum of not to exceed ten seconds. The film formed willinitially be quite uniform in thickness and characterized by the absenceof large bubbles. These, however, will form gradually and should bedestroyed by actuation of the motor 34 and tumbling of the parts tolargely destroy the film before damage to the finish occurs. The lengthof time the barrel should remain motionless will vary from a minimum ofabout ten seconds to a maximum of about two hundred seconds. The time isnecessarily variable and is influenced not only by the character of thebath but also its temperature and the density of the current flowingbetween the parts and the cathode. The appropriate intervals may beselected by observation in each instance.

The timer is adjusted so that rotation of the motor will occur beforegas bubbles of appreciable size are formed on the surface being polishedand before such bubbles break free from the parts and rise to thesurface of the bath in appreciable quantity. Actuation of the motorcauses rotation of the barrel and the desired tumbling of the containedparts, this action being aided by the strips 37 which serve as flightsto insure movement of the small parts in the desired manner. The periodof barrel rotation may be quite short and sutficient only to destroy thegas film and expose a new surface to the action of the cathode. Thiswill ordinarily occur in approximately one-quarter of a revolution, butgreater movement is not harmful and several revolutions may be utilizedif necessary to thoroughly disperse the film. This procedure is, ofcourse, repeated until a satisfactory surface finish is obtained. Uponcompletion of the polishing operation parts may be removed from thebarrel 22, washed and packed for shipment.

It should not be understood from the foregoing that the materials ofconstruction indicated are intended to limit my invention. Theyconstitute satisfactory materials for the use described but othermaterials may be substituted. Titanium constitutes a preferred materialfor portions of the electrical circuit constituting the anode but shouldnot be employed as a cathode. The cathode may be formed of copper,carbon or other suitable material, copper being the most generallyavailable. It will also be understood that other driving mechanisms maybe utilized as, for example, gears instead of belts.

By following the method of operation herein described, the time requiredto impart the desired lustrous finish to the parts being treated will bereduced and the charatcer of the finish materially improved.Furthermore, it has been found that small objects such as screws treatedas herein described exhibit improved corrosion resistance when tested ina salt spray.

I claim:

1. The method of enhancing the uniformity of lustre of small metalobjects capable of being electropolished in an electrolytic bath whichincludes the steps of arranging a plurality of said objects in contactwith each other in a relatively thin generally rectangular zone in anelectropolishing electrolytic bath, maintaining said objects motionlesswhile passing electrical current from said parts to a cathode spacedfrom said zone, thereby forming a gas film on the surfaces of the partsexposed directly to said cathode, maintaining said objects motionlessuntil said gas film consists of a multiplicity of bubbles of irregularsize, then destroying said film by tumbling said objects within saidzone and thereby exposing a substantially bubble free surface to thecathode, and periodically repeating said steps of holding said objectsmotionless and tumbling said objects to destroy a film of bubbles ofirregular size while continuing the passage of electrical currentbetween the parts and the cathode.

2. The method defined in claim 1 which'includes the step of maintainingsaid parts motionless for not less than ten seconds.

3. The method defined in claim 1 which includes the steps of maintainingsaid parts motionless for a period of not more than two hundred seconds.

4. The method defined in claim 1 wherein said parts are maintainedmotionless for not less than ten seconds nor more than two hundredseconds.

References Cited by the Examiner UNITED STATES PATENTS 1,873,826 8/1932Devecis 204213 2,390,282 12/1945 Tour et al 204- 2,741,463 4/ 1956C-olclesser 204213 X 3,058,902 10/1962 Neilson 204213 3,213,008 10/1965Valentin 204-1405 OTHER REFERENCES Ransohoif, Plating, November 1956,pages 3146-8.

JOHN H. MACK, Primary Examiner.

HOWARD S. WILLIAMS, Examiner.

R. MIHALEK, Assistant Examiner.

1. THE METHOD OF ENHANCING THE UNIFORMITY OF LUSTRE OF SMALL METALOBJECTS CAPABLE OF BEING ELECTROPOLISHED IN AN ELECTROLYTIC BATH WHICHINCLUDES THE STEPS OF ARRANGING A PLURALITY OF SAID OBJECTS IN CONTACTWITH EACH OTHER IN A RELATIVELY THIN GENERALLY RECTANGULAR ZONE IN ANELECTROPOLISHING ELECTROLYTIC BATH, MAINTAINING SAID OBJECTS MOTIONLESSWHILE PASSING ELECTRICAL CURRENT FROM SAID PARTS TO A CATHODE SPACEDFROM SAID ZONE, THEREBY FORMING A GAS FILM ON THE SURFACES OF THE PARTSEXPOSED DIRECTLY TO SAID CATHODE, MAINTAINING SAID OBJECTS MOTIONLESUNTIL SAID GAS FILM CONSISTS OF A MULTIPLICITY OF BUBBLES OF IR-